1. Field of the Invention
The present invention relates generally to the technical field of amplifiers employed in high frequency bands, and more particularly to a device and method for correcting the voltage control signals of the amplifiers.
2. Description of the Related Art
In a linear transmitter, a signal to be transmitted is linearly amplified and transmitted. In order to linearly amplify signals of various (power) levels ranging from low to high, it is necessary to supply power to the amplifier based on a high level signal. However, constant application of high supply voltage to the amplifier has the problem of significant reduction in amplification efficiency in the case of amplifying a small (low power) signal. This is disadvantageous in particular to simple mobile communications devices and devices using a small-size battery. In order to cope with such a problem, the technique of suitably switching supply voltage to be applied to an amplifier in accordance with the level of a signal to be amplified is proposed (see, for example, Japanese Laid-Open Patent Application No. 3-174810).
FIG. 1 is a diagram for illustrating such amplification control. A transmission signal is input to an amplifier 101. The amplifier 101 amplifies the input signal in accordance with a voltage control signal v, and outputs an amplified output signal. The voltage control signal v varies in accordance with the amplitude level of the transmission signal, and is derived from an envelope detector 102. A signal indicating the level of amplitude may be referred to as “envelope signal.”
FIG. 2 is a graph showing exemplary input/output characteristics of an amplifier. The horizontal axis represents the voltage level of an input signal, and the vertical axis represents the voltage level of an output signal. In FIG. 2, three sets of input/output characteristics are indicated by respective lines a, b, and c. The line a shows the case where the input/output characteristic shows linearity at or below an input voltage a1 and is non-linear above the input voltage a1. The line b shows the case where the input/output characteristic shows linearity at or below an input voltage a2 and is non-linear above the input voltage a2. The line c shows the case where the input/output characteristic shows linearity at or below an input voltage a3 and is non-linear above the input voltage a3.
The envelope detector 102 of FIG. 1 measures the level of the transmission signal. If the measured level is low, the envelope detector 102 sets the voltage control signal v for the amplifier 101 to v1 (v=v1). As a result, the amplifier 101 shows an input/output characteristic as indicated by a in FIG. 2, and small signals having a level lower than or equal to v1 are linearly amplified. On the other hand, if the measured level of the transmission signal is high, the envelope detector 102 sets the voltage control signal v for the amplifier 101 to v3 (v=v3). As a result, the amplifier 101 shows an input/output characteristic as indicated by c in FIG. 2, and large signals are also linearly amplified. Thus, by suitably changing the supply voltage of the amplifier 101 in accordance with the input signal, it is possible to obtain a linearly amplified output signal with high efficiency. Practically, voltages of more than these three levels v1, v2, and v3 are applied continuously or step by step to the amplifier 101.
According to the above-described method, supply voltage is suitably changed in accordance with the level of the input signal. Accordingly, the input signal and the voltage control signal v should be timed appropriately with each other. On the other hand, the properties of devices (the properties of analog devices in particular) vary to some extent because of the material characteristics of the devices, manufacturing processes, and manufacturing environment. As a result, there is concern that the transmission signal (input signal) and the voltage control signal v are slightly out of phase with each other.
FIG. 3 shows the input signal, output signal, and voltage control signal of an amplifier. The horizontal axis represents time, and the vertical axis represents an amplitude level. FIG. 3 shows the case where the input signal and the voltage control signal, which should be in phase with each other, are out of phase with each other by a time τ, so that the waveform of the output signal is different from what it should be. For example, in the period indicated by T1, the fed voltage control signal is greater than the input signal. In this case, the input signal itself may be linearly amplified. However, since a voltage higher than the necessary supply voltage is applied to the amplifier, the efficiency of amplification is reduced. In the period indicated by T2, a signal of a level exceeding the highest voltage that can be linearly amplified by the amplifier is input to the amplifier. Accordingly, the output signal of the amplifier deviates from the linear amplification of the input signal, and is non-linearly amplified. Thus, if the input signal and the voltage control signal are not appropriately timed with each other, the degradation of a signal and the radiation of unnecessary waves are caused. This problem of timing offset occurs product by product. Accordingly, its compensation should also be performed product by product. However, a method for automatically performing such compensation or control with efficiency has not been discovered, while performing it manually is time-consuming and unsuitable for controlling many products.